Method of pre-processing digital images, and digital image preprocessing system

ABSTRACT

A method of pre-processing digital images captured by an image sensor for encoding is disclosed. The method comprises receiving a first digital image, receiving information representative of a movement of the image sensor at a time of capture of said first digital image), and pre-processing said first digital image. At least one parameter of said pre-processing is dependent on said information representative of movement. A digital image pre-processing system is also disclosed.

FIELD OF INVENTION

The present invention relates to the field of digital image processing,and more particularly to pre-processing of digital images in preparationfor encoding.

BACKGROUND

In many uses of digital images, e.g., in monitoring systems, the digitalimages are compressed using various encoding schemes for transmission orstorage. Before encoding, pre-processing of the digital images isperformed. Pre-processing may for instance involve sharpening or noisefiltering, in order to improve the quality of the images. However, aproblem arises when the image sensor capturing the images moves duringcapture. Firstly, the ensuing changes in the images lead to a higheroutput bit rate from the encoder, because there is less temporalredundancy between the images. If bandwidth is very restricted, such aswhen transmitting over a mobile network, it may even be impossible totransmit video from a camera when fast movement causes increasedbitrate. Additionally, the movement may make it computationally heavierto pre-process the image. On top of that, the pre-processing may add tothe number of bits required for representing the images compared to acorresponding image captured when the image sensor is stationary. Onesolution to this problem is to freeze encoding of images during panningand/or tilting of a camera comprising the image sensor, and to resumeencoding new input image frames only after movement has ceased. However,that means visual information is lost during movement, and an operatorwatching the transmitted image sequence may be disoriented. Thus, thereis a need for an improved encoding approach.

SUMMARY

It is an object of the present invention to provide a method ofpre-processing digital images for encoding, which allows a lower bitratewhen the image sensor capturing the images is moving. Another object ofthe invention is to provide a method of pre-processing which makes itpossible to avoid unnecessarily heavy computations when the image sensoris moving. A further object is to provide a method of pre-processingdigital images for encoding, which enables visual information alsoduring movement of the image sensor. Still another object is to providea pre-processing method that may provide images with improved usabilityduring movement of the image sensor.

It is also an object of the invention to provide a pre-processing systemthat may avoid unnecessarily high bitrate and unnecessarily highcomputational usage when the image sensor capturing the images ismoving. Further, it is an object of the invention to provide apre-processing system which may enable improved usability of imagesduring movement of the image sensor.

An additional object is to provide a pre-processing system enablingprovision of images also when the image sensor is moving.

Yet another object is to provide a computer program product that enablesefficient bitrate also when the image sensor capturing the images ismoving.

According to a first aspect, these objects are achieved, in full or atleast in part, by a method of pre-processing digital images captured byan image sensor for encoding, comprising: receiving a first digitalimage, receiving information representative of a movement of the imagesensor at a time of capture of said first digital image, andpre-processing said first digital image, wherein at least one parameterof said pre-processing is dependent on said information representativeof movement. In this manner, pre-processing may be adapted such that themovement of the image sensor does not cause unnecessarily high bitrate.Thereby, images may be encoded and stored or transmitted for displayingalso during periods of movement of the image sensor, without the cost ofincreased bitrate. Further, since the images captured during movement ofthe image sensor, e.g., during a fast panning movement, will generallybe blurry, a loss of image quality possibly caused by modifying thepre-processing will have little impact on the usability of such images.

If the movement of the image sensor is above a predetermined threshold,said at least one parameter of said pre-processing may be modified suchthat the first digital image is less processed by said pre-processingthan if the movement were equal to or below said threshold.

Further, said at least one parameter of said pre-processing may bemodified such that at least one pre-processing procedure is disabled.For instance, sharpening may be dispensed with when the image sensor ismoving, as the images would not really benefit from sharpening anyway.

According to another variant, said at least one parameter is modifiedsuch that at least one pre-processing procedure processes said firstdigital image, but to a greater or lesser extent. As an example,filtering may be applied to a greater degree. As a further example, tonemapping may be performed, but to a lesser degree.

According to yet another variant, said at least one parameter of saidpre-processing is modified such that at least one pre-processingprocedure that is otherwise disabled is enabled. For instance, blurringmay be applied during movement of the image sensor, thereby makingblocks of pixels in the images more or less homogeneous, which leads tolower bit requirement for representation of the image.

The pre-processing may comprise performing at least one pre-processingprocedure chosen from the group consisting of sharpening, addingcontrast, tone mapping, colour correction, spatial and/or temporal noisefiltering, blurring, Bayer pattern sampling, demosaicing, HDR merging,controlling focus, controlling exposure, and controlling gain. It shouldbe noted that, in the context of image sensor data, noise is sensor datathat represents something that was not present in the scene. Noise maybe temporal, i.e., varying from one image frame to the next, or spatial,i.e. differing from one pixel to another within the same frame. Thereare various sources of noise. Temporal noise may, e.g., be reset noise,thermal noise, flicker noise, dark current shot noise, quantisationnoise or phase noise. Spatial noise may, e.g., be dark fixed-patternnoise, light fixed-pattern noise, leaker noise, defect pixel noise orcosmetic defect noise.

The information representative of a movement of the image sensor may beprovided by a pan, tilt and/or zoom controller controlling movement ofsaid image sensor. In this manner, adaptation of the pre-processing maybe made in view of intentional movement of the image. It may here benoted, that zooming does not in itself imply a movement of the imagesensor. However, for purposes of encoding images, zooming will inducechanges in the captured images analogous to actual movement of the imagesensor. Therefore, in the context of this application, a zoom operationwill be seen as equivalent to a movement of the image sensor.

The information representative of a movement of the image sensor may beprovided by a motion sensor. Hereby, undesired movement caused byshaking may be taken into account. Information from the motion sensormay also be used for providing information on pan, tilt and/or zoomoperations.

According to a variant of the method, said at least one parameter ismodified gradually in preparation for a planned movement of the imagesensor. In this manner, e.g., harder noise filtering may be addedgradually leading up to a planned movement, or sharpening may begradually decreased. Thereby, abrupt changes in the appearance of theimages may be avoided.

The method may further comprise encoding said first digital image as anintra-frame in preparation for a planned movement of the image sensor.If there is movement in the captured images, inter-frames will becomelarger, due to larger differences between frames. At some point, thedifferences will be large enough that an intra-frame, not relying onother image frames for its encoding, will be more efficient. If it isknown that there will be movement, an intra-frame can be forced, suchthat unnecessarily large inter-frames may be avoided. By forcing, orinserting, an intra-frame, the encoded frame may have higher quality, asmay subsequent images encoded as inter-frames with reference to thatintra-frame. The subsequent inter-frames may also require fewer bitsbecause of having a more suitable reference frame.

According to a second aspect, the above-mentioned objects are achieved,in full or at least in part, by a digital image pre-processing systemfor pre-processing input images captured by an image sensor forencoding, the system comprising: an image receiving module arranged toreceive input images, a movement information receiving module arrangedto receive information representative of a movement of the image sensorat a time of capture of said first digital image, and a pre-processingmodule arranged to pre-process the input images before encoding, whereinat least one parameter of said pre-processing is dependent on saidinformation representative of movement. By means of such a system, itmay be possible to provide images during movement of the image sensor,without the cost of unnecessarily high output bitrates.

The pre-processing module may be arranged to perform at least onepre-processing procedure chosen from the group consisting of sharpening,adding contrast, tone mapping, colour correction, temporal and/orspatial noise filtering, blurring, Bayer pattern sampling, demosaicing,HDR merging, controlling focus, controlling exposure, and controllinggain.

In an embodiment of the digital image pre-processing, the movementinformation receiving module is communicatively connected to a pan, tiltand/or zoom controller controlling movement of said image sensor.Thereby, information on intentional movement may be provided.

Alternatively or additionally, the movement information receiving modulemay be communicatively connected to a motion sensor. As discussed inconnection with the method, information on unintentional as well asintentional movement may thereby be provided.

According to a third aspect, the above-mentioned objects are achieved,in full or at least in part, by a camera comprising a pre-processingsystem according to the second aspect. Such a camera may generally beembodied in the same ways as the second aspect, with accompanyingadvantages.

According to a fourth aspect, the above-mentioned objects are achieved,in full or at least in part, by a computer program product comprising acomputer-readable storage medium with instructions adapted to carry outthe method according to the first aspect when executed by a processor.The processor may be any kind of processor, e.g., a central processingunit (CPU), a graphics processing unit (GPU), a custom made processingdevice implemented in an integrated circuit, an ASIC, an FPGA, orlogical circuitry including discrete components. The computer-programproduct may generally be varied in the same ways as the first aspectwith accompanying advantages.

A further scope of applicability of the present invention will becomeapparent from the detailed description given below. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thescope of the invention will become apparent to those skilled in the artfrom this detailed description.

Hence, it is to be understood that this invention is not limited to theparticular component parts of the device described or steps of themethods described as such device and method may vary. It is also to beunderstood that the terminology used herein is for purpose of describingparticular embodiments only, and is not intended to be limiting. It mustbe noted that, as used in the specification and the appended claim, thearticles “a”, “an”, “the”, and “said” are intended to mean that thereare one or more of the elements unless the context clearly dictatesotherwise. Thus, for example, a reference to “an object” or “the object”may include several objects, and the like. Furthermore, the word“comprising” does not exclude other elements or steps.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention will now be described in more detail by way of example andwith reference to the accompanying schematic drawings, in which:

FIG. 1 is an illustration of a scene being monitored by a camera,

FIG. 2 is a front view of a camera with pan/tilt/zoom capability,

FIG. 3 is a block diagram of the camera of FIG. 2,

FIG. 4 is a block diagram showing some functions in a process ofcapturing, pre-processing and encoding images,

FIG. 5 is a flow chart of a method of pre-processing digital images,

FIG. 6 is a block diagram of an embodiment of a digital imagepre-processing system, and

FIG. 7 is a block diagram of a camera incorporating a digital imagepre-processing system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In FIG. 1, a scene 1 is shown, which is monitored by a camera 2. Anexample of a camera 2 is shown in FIG. 2. This camera 2 has an imagesensor (not shown in FIG. 2) and a lens 3. The camera 2 is a so calledpan-tilt-zoom (PTZ) camera with capability to pan around a pan axis 4and to tilt around a tilt axis 5. The camera 2 is also capable ofzooming.

FIG. 3 is a block diagram showing some of the components of the camera2. Here, an image sensor 6 arranged to capture images may be seen, aswell as a PT controller 7 for controlling panning and tilting of thecamera 2. Further, the camera 2 has a zoom controller 8 for controllingzooming. The camera 2 will also have additional components, known in theart, which are not relevant for the invention, and which will thereforenot be shown or discussed here.

The basic principles of a process for capturing and encoding images ofthe scene 1 with the camera 2 will now be described with reference toFIG. 4. Light from the scene 1 enters the camera 2 through the lens 3and is captured 10 by the image sensor. The data from the image sensor 6is in the form of grey scale intensities, generally with each pixelbeing represented by a respective digital value. The captured image, ormore specifically the data representing the image, then undergoespre-processing 11. Pre-processing 11 may involve one or morepre-processing procedures, such as Bayer pattern sampling, demosaicing,sharpening and tone mapping. The pre-processing procedures generally aimat providing a higher quality or more usable image. The pre-processedimage is thereafter ready for encoding 12. Encoding may be doneaccording to any compression format.

If the camera 2 moves during capture of a video sequence, be it becauseof a planned or spontaneous intentional panning, tilting or zoomingoperation or because of unintentional vibrations, the images may becomeblurry. Further, the movement leads to increased bitrates forrepresenting images, since there is less redundancy between images of amoving scene than between images of a static scene. The increasedbitrate may be too high for transmitting video if bandwidth is veryrestricted, such as on a mobile network, thereby making it impossible totransmit any images, e.g., during fast panning or tilting movements ofthe camera. As mentioned above, the bitrate problem is sometimes dealtwith by freezing an image during panning, tilting and/or zooming. Inthis manner, no images are pre-processed and encoded during movement,thereby leading to a lower bitrate. However, such an approach means thatthe operator is provided with less or no visual information duringmovement. The operator therefore has little chance of noting suspiciousobjects or behaviour occurring in areas of the scene that aretransitioned during movement. The lack of visual information duringmovement may also lead to disorientation, since the operator is onlyprovided with images of the start position of the movement and of theend position of the movement, and none there between. These problems arethe same for live display of video sequences as for recorded videosequences.

According to the invention, images should not be frozen during movementof the camera. Although this may lead to higher bitrate, the visualinformation available during movement may be worth it. However, theinventors have realised that part of the bitrate increase may be avoidedby modifying the pre-processing applied before encoding.

For instance, sharpening works by increasing differences in the image,may thereby also lead to increased noise. The differences and noise, inturn, lead to higher bitrates when encoding the sharpened image. Byreducing or even disabling sharpening during movement of the camera, ormore precisely the image sensor, the increase in bit rate may be curbed.

Similarly, tone mapping used for improving the dynamic range of an imageadds differences in the bright and dark areas of the image and may addnoise, thereby increasing bitrate. Disabling tone mapping duringmovement may inhibit part of the bitrate increase.

In the same way, other pre-processing procedures that would otherwiseadd to the bitrate may be disabled or applied to a lesser extent.Intuitively, this will lower the image quality, but since imagescaptured during movement of the camera are generally blurry, the loweredimage quality will in most cases not be of any concern to the operator.Further, during fast movement, the images may not even benefit frompre-processing.

Reduction or disabling of sharpening may even be taken as far asactually adding blur during movement of the camera. Thus, instead ofincreasing differences in the image, differences may be reduced, e.g.,by calculating an average luminance value of a block or group of pixelsand using that as the luminance value of all pixels in that block. Inthis manner pixel values are “smeared” over areas of the image.

Some pre-processing procedures may be modified during movement of thecamera although not thereby contributing to the goal of loweringbitrate, but actually improving image quality or image usability. Somecameras improve the dynamic range in the images by capturing twosuccessive images within a short period of time. For one of the images alow exposure is used, resulting in an image where bright areas of thescene may be scene clearly, but where dark areas lack details and aremore or less completely black. For the other image a high exposure isused, resulting in an image where the bright areas are overexposed orsaturated, appearing completely white, and where the dark areas showdetails. By selectively using areas of one or the other image, a mergedimage is produced in which both dark and bright areas of the scene maybe shown in detail. Such pre-processing procedures are referred to asHDR (high dynamic range) merging or WDR (wide dynamic range) merging.For such merging to give a good result, the two differently exposedimages have to be captured with such short time between them that theyshow the same scene, or else objects moving in the scene between the twocaptures will appear blurry. However, if the camera is moving it mightnot be possible to capture the two images sufficiently close together intime for them to actually show the same scene. The entire image maytherefore appear blurry. In order to remedy this problem, HDR mergingmay be disabled during movement of the camera. Thus, only one of the twodifferently exposed images may be encoded, or capturing of twodifferently exposed images may be disabled, such that just one image iscaptured and encoded.

Image or video statistics, such as luminance values, chromaticityvalues, variance of luminance and/or chromaticity, motion information,autofocus values, moving object counter, bitrate, or filter parameters,gathered over time during a video sequence, may be reset when movementof the image sensor is determined. In this way, the movement will notcorrupt the statistics.

In order to ascertain that a modification of one or more of thepre-processing procedures should be made, information on movement of thecamera 2, or more precisely the image sensor 6, is obtained. Thisinformation may be provided by the PT controller 7, which has thisinformation anyway for controlling panning and tilting of the camera 2.Similarly, such information may be provided by the zoom controller 8.The camera 2 may also have a motion sensor 9, e.g., an accelerometer,which senses movement of the image sensor. The motion sensor 9 mayprovide information on movement of the image sensor 6 when the camera 2is unintentionally vibrating or shaking. It may also provide informationon intentional movement of the image sensor 6 in addition to or insteadof the PT controller 7.

As already discussed, the movement may be a desired movement or anundesired movement. Desired movements may, e.g., be controlled using aninput device such as a joystick. Desired movements may also be plannedand pre-programmed as so called guard tours. Undesired movements may,e.g., be vibrations caused by wind or traffic, or by insufficient fixingof the camera.

If a movement of the camera is planned, it is possible to makemodifications of one or more pre-processing procedures in preparationfor the movement. For instance, temporal noise filtering may be modifiedsuch that filtering is gradually made more severe, and may then be keptat an increased level until the end of the movement. An increased noisefiltering may lead to loss of details in the pre-processed image, butagain, during movement of the camera, this will in most cases not be ofconcern for the usability of the images. Filtering may be donetemporally as well as spatially. One or both of these two types may beused, and a balance there between may be modified during movement, suchthat more or only temporal filtering is done when the image sensor isstationary, and more or only spatial filtering is done when the imagesensor is moving.

In preparation of a planned movement, encoding may also be modified. Ifit is known that the camera will move, it is implicitly known that therewill be apparent movement in images, leading to larger inter-frames.When movement starts, an intra-frame may therefore be forced, therebyproviding a better reference for subsequent inter-frames.

With reference to FIG. 5, the method will now be described in moregeneral terms. A first digital image is received (step S01) andinformation representative of a movement of the image sensor at a timeof capture of the first image is received (step S02). The first digitalimage is pre-processed (step S03). Based on the informationrepresentative of movement, it is decided if and how one or moreparameters of the pre-processing is to be modified (step S04). Forinstance, the movement may be compared to a predetermined threshold, andif the movement is above that threshold, one or more pre-processingprocedures are disabled, enabled or otherwise modified.

The pre-processing may be modified based on information representativeof movement received for a previous image, such that a movement of theimage sensor will affect the pre-processing only of subsequentlycaptured images. For some pre-processing procedures, parameters may bemodified immediately, such that a movement of the image sensor willaffect the pre-processing of the image for which the informationrepresentative of movement of the image sensor was received, andpossibly also of subsequently captured images.

After pre-processing (S03), the first image is encoded (step S05), suchthat it may be transmitted and/or stored.

In FIG. 6 an embodiment of a digital image pre-processing system 20 isshown. The system 20 has an image receiving module 21, which is arrangedto receive input images, and a movement information receiving module 22,which is arranged to receive information representative of a movement ofthe image sensor at a time of capture of the input images. Further, thesystem 20 has a pre-processing module 23 arranged to pre-process theinput images. At least one parameter of the pre-processing is dependenton the information representative of movement of the image sensor. Afterpre-processing, the images may be encoded by an encoder (not shown). Theencoder may be integrated in the pre-processing system 20, or it may beseparate from the pre-processing system 20. The pre-processing system 20may be integrated in a camera, such as the camera 102 schematicallyshown in FIG. 7. In FIG. 7, components of the camera 102 that are thesame as in the camera 2 shown in FIG. 3 are indicated with the samereference numerals, but with the addition of one hundred, and will notbe discussed further here.

Alternatively, the pre-processing system 20 may be a separate unit,operatively connected to a camera, such as the camera 2 in FIG. 3.

The pre-processing system may be embodied as hardware, firmware,software, or a combination thereof.

It will be appreciated that a person skilled in the art can modify theabove described embodiments in many ways and still use the advantages ofthe invention as shown in the embodiments above. As an example, a coupleof pre-processing procedures have been mentioned, but the invention isequally applicable to other pre-processing procedures applied inpreparation for encoding a digital image, e.g., adding contrast, colourcorrection, Bayer pattern sampling, demosaicing or controlling gain. Itwill be understood that pre-processing procedures that tend to add noisemay be reduced or disabled in order to lower the output bitrate, whereaspre-processing procedures that tend to reduce noise may be enabled orincreased to the same end.

In connection with FIG. 3, the PTZ controller is described as onecontroller for panning and tilting and one for zooming. However, it mayas well be made up of separate controllers for each of panning, tiltingand zooming, respectively, or of one common controller for panning,tilting and zooming.

The predetermined threshold against which the movement of the imagesensor is compared may be pre-programmed or set by user input.

Instead of using just one threshold, a plurality of thresholds may beused. For instance, if movement is above a first, low threshold, one ormore pre-processing procedures may be reduced, and if movement is abovea second, higher threshold, these pre-processing procedures may bedisabled. A parameter of the pre-processing may depend on the movement,e.g., as a linear function. Thus, with increasing movement, theparameter may be gradually decreased. As an example, if the camera isslowly panning or tilting, sharpening may be slightly reduced, and ifthe camera is moving more quickly, sharpening may be more severelyreduced or even disabled, the sharpening thus being dependent on themovement level.

The pre-processing system may be embodied as software, firmware,hardware, or a combination thereof.

The pre-processing module of the pre-processing system may be arrangedto perform all the pre-processing procedures to be applied on theimages. Alternatively, the pre-processing module may be divided intoseveral submodules, independent or cooperating, each performing just oneor a few pre-processing procedures.

In the examples above, the invention is described in connection with acamera. The camera may be a monitoring camera. Further, the camera maybe any type of camera, e.g., a camera employing visible light, an IRcamera or a thermal camera. The camera may be a digital camera, but theinvention may also be used with analogue cameras. In such case, imagesfrom an analogue camera may be converted to digital format using adigitalisation unit.

Instead of in a camera, the image sensor capturing the image frames maybe arranged in another type of image capturing device.

The images may also be generated by a visual light sensor, a thermalsensor, a time-of-flight sensor, or other types of image generatingsensors.

Thus, the invention should not be limited to the shown embodiments butshould only be defined by the appended claims.

What is claimed is:
 1. A method of pre-processing digital imagescaptured by an image sensor for encoding, comprising: receiving a firstdigital image, receiving information representative of a movement of theimage sensor at a time of capture of said first digital image, modifyingat least one parameter of pre-processing of the digital image based onsaid information representative of movement, pre-processing said firstdigital image according to the at least one modified parameter togenerate a pre-processed image, wherein the modifying causes a bitrateof the pre-processed image when encoded to be lower than a bitrate of apre-processed image that would be generated by applying thepre-processing without the modified parameter and when encoded.
 2. Themethod according to claim 1, wherein if said movement of the imagesensor is above a predetermined threshold, said at least one parameterof said pre-processing is modified such that the first digital image isless processed by said pre-processing than if said movement were equalto or below said threshold.
 3. The method according to claim 2, whereinsaid at least one parameter of said pre-processing is modified such thatat least one pre-processing procedure is disabled.
 4. The methodaccording to claim 2, wherein said at least one parameter is modifiedsuch that at least one pre-processing procedure processes said firstdigital image, but to a greater or lesser extent.
 5. The methodaccording to claim 1, wherein said at least one parameter of saidpre-processing is modified such that at least one pre-processingprocedure that is otherwise disabled is enabled.
 6. The method accordingto claim 1, wherein said pre-processing comprises performing at leastone pre-processing procedure chosen from the group consisting ofsharpening, adding contrast, tone mapping, colour correction, noisefiltering, blurring, Bayer pattern sampling, demosaicing, HDR merging,controlling focus, controlling exposure, and controlling gain.
 7. Themethod according to claim 1, wherein said information representative ofa movement of the image sensor is provided by a pan, tilt and/or zoomcontroller controlling movement of said image sensor.
 8. The methodaccording to claim 1, wherein said information representative of amovement of the image sensor is provided by a motion sensor.
 9. Themethod according to claim 1, wherein said at least one parameter ismodified over a plurality of frames in preparation for a plannedmovement of the image sensor.
 10. The method according to claim 1,further comprising encoding said first digital image as an intra-framein preparation for a planned movement of the image sensor.
 11. Acomputer program product comprising a computer-readable storage mediumwith instructions adapted to carry out the method according to claim 1when executed by a processor.
 12. A digital image pre-processing systemfor pre-processing input images captured by an image sensor forencoding, the system comprising: an image receiving module arranged toreceive input images; a movement information receiving module arrangedto receive information representative of a movement of the image sensorat a time of capture of said input images; and a pre-processing modulearranged to modify at least one parameter of pre-processing of the inputimages based on said information representative of movement, and topre-process the input images before encoding to generate pre-processedimages, wherein the modifying causes bitrates of the pre-processedimages when encoded to be lower than bitrates of pre-processed imagesthat would be generated by applying the pre-processing without themodified parameter and when encoded.
 13. The digital imagepre-processing system according to claim 12, wherein said pre-processingmodule is arranged to perform at least one pre-processing procedurechosen from the group consisting of sharpening, adding contrast, tonemapping, colour correction, noise filtering, blurring, Bayer patternsampling, demosaicing, HDR merging, controlling focus, controllingexposure, and controlling gain.
 14. The digital image pre-processingsystem according to claim 12, wherein said movement informationreceiving module is communicatively connected to a pan, tilt and/or zoomcontroller controlling movement of said image sensor and/or to a motionsensor.
 15. A camera comprising a pre-processing system according toclaim 12.